Lubrication free connection

ABSTRACT

A tenter clip has a jaw pivot shaft having a pair of molded bearing surfaces of superior strength, wear resistance and stability under high temperature operating conditions. The method for manufacture of the jaw pivot shaft includes knurling recesses formed in the shaft prior to applying plastic bearing material to the recesses by injection molding and finishing machining the pin by centerless grinding.

TECHNICAL FIELD

This invention relates to the construction of a self lubricating shaft or pivot pin suitable for operating at high temperatures such as in tenter frame apparatus.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 5,067,214 issued Nov. 26, 1991 to Christopher E. Hosmer and John F. Whaley for a Tenter Framer Apparatus and Method shows a self-lubricating bushing made of a polyamide resin.

U.S. Pat. No. 5,797,172 issued Aug. 25, 1998 to Christopher Eugene Hosmer for a Tenter Frame and Method discloses tenter frame apparatus of the type in which this invention has particular application.

Conventional self lubricating friction or wear devices such as bushings and/or plain bearings are typically composed of a metal housing in which a plain plastic bushing is inserted and secured in a manner preventing rotation of the bushing. Typically a metal shaft is carried in the interior diameter of the bushing. In the case of a tenter frame clip jaw application the currently issued bushings result in the bushing locking down on the metal shaft due to thermal expansion and the bushing wears out prematurely or it locks up preventing the desired function of the jaw.

BRIEF SUMMARY OF THE INVENTION

Attachment of the self lubricating plastic bushing is achieved by insert injection molding, at high pressure and temperature, a plastic sleeve around the metal pin. To enhance adhesion and provide sufficient material for wear, the plastic is molded in a groove of a specific depth which has a knurled surface. The self lubricating bearing member is thus on the shaft which rotates in the metal housing. With this design, thermal expansion at elevated temperature results in contraction of the internal diameter of the plastic sleeve. Because the internal diameter cannot decrease, the forces holding the plastic sleeve on the pin are increased thereby preventing the sleeve bearing from loosening on the metal pin. To minimize the outside diameter expansion of the plastic bearing sleeve, a low coefficient of thermal expansion material with good self lubricating properties is used.

The process by which the hybrid steel and plastic pin is produced is also unique and very cost effective. Although CNC turning of the hybrid pin to the desired outside diameter and dimensional tolerance is possible, a more cost effective and dimensionally and surface finish superior part is produced by centerless grinding the outside diameter. The process of centerless grinding metal and plastic simultaneously is unique and requires a plastic that is dimensionally stable and rigid enough to grind properly and not melt and smear or have metal particles inbedded.

A polytetraflouraethylene is preferred for use in tenter frame jaw pins for its physical, thermal and tribological properties which are required to achieve a long wearing, stable product with reliable performance.

BRIEF DESCRIPTION OF THE DRAWINGS

One embodiment of the invention is illustrated in the drawings in which:

FIG. 1 is a perspective view of a tenter frame with parts separated and broken away for illustration purposes;

FIG. 2 is an end view of the tenter frame in an assembled condition;

FIG. 3 is a side view of a metal pivot shaft initially machined to provide two reduced diameter recesses;

FIG. 4 is a side view of the shaft after the recessed areas of the shaft have been knurled;

FIG. 5 is a side view of the shaft after a plastic material has been applied to the recessed areas by injection molding;

FIG. 6 is a side view of the shaft after it has been machined by centerless grinding;

FIG. 7 is a section view taken along the line VII-VII in FIG. 6;

FIG. 8 is a side view of a centerless grinding apparatus, and

FIG. 9 is a top view of the apparatus shown in FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2, the invention is illustrated in a tenter clip 11 which with other clips, are connected in series by a link chain 12 thus providing a tenter chain 13. Tenter chains are provided in opposing pairs to grip the edge of a continuous web of flat material such as woven cloth and to simultaneously stretch the material in a transverse direction while conveying it through a process oven as a finishing operation to its manufacture.

The tenter clip 11 is a clamping device which includes a clip body 14 which is open on one side with a surface 16 for supporting web material being fed into it. The clip body 14 includes a pair of support arms 17, 18 pivotally supporting a pivot pin 21 for a jaw 22. The pivotable jaw 22 is operated by an external mechanism, not shown, to allow entry and exit of the web material. The jaw 22, and the clip 11, are designed to automatically clamp and hold the web material. The jaws movement is a limited pivotal movement about the axis 23 of the jaw pivot pin 21 which is nonrotatably secured to the jaw 22 by a transverse pin 24 extending through aligned bores in the pin 21 and the jaw 22.

Referring to FIGS. 3-7 the process for manufacture of the jaw pivot pin 22 is illustrated by showing the steps of manufacture.

FIG. 3 shows the pin 22 after the pin is rough finished to a predetermined oversize diameter and a pair of reduced diameter recesses 31, 32 are formed near its axially opposite ends. The recesses 31, 32 having an axial dimension matching the bearing surfaces of the arms 17, 18 of the tender clip 11.

FIG. 4 shows the pin 22 after the reduced diameter surface of the recesses 31, 32 have been knurled.

FIG. 5 shows the pin 22 after sleeves 34, 36 of high temperature resistant plastic, such as polytetraflouraethylene has been applied by injection molding to the reduced diameter areas 31, 32 of the pin 22. The oversized pin 22 is next machined by centerless grinding to a desired finished diameter as illustrated in FIGS. 6 and 7.

FIGS. 8 and 9 illustrate the centerless grinding step. The shaft 22 is supported by a work blade 41 and is sandwiched between a grinding wheel 42 and a regulating wheel 43. As shown in FIG. 9 the pin 22 is guided by work rest guides 46, 47, 48, 49 during a machining operation. 

1. A method of manufacturing a pivot pin for use in a high temperature environment comprising the steps of: machining a pin to a predetermined oversize diameter machining a pair of axially spaced relatively shallow reduced diameter recesses in said pin of an axial width suitable for registering with a pair a bearing surfaces of a member to which the pin is to be attached, roughing up said recesses, applying a cylindrical bearing sleeve of self lubricating thermoplastic compound to each of said recesses by injection molding, and finishing machining the outside diameter of said pin including said bearing sleeves to a precise finished diameter by centerless grinding.
 2. The method of claim 1 wherein said thermoplastic compound is polytetraflouraethylene.
 3. The method of claim 1 wherein said thermoplastic compound has a melting point above 600 degrees F.
 4. A tenter clip comprising: a clip body having an open side with a flat surface for supporting web material being fed into said open side and a pair arms spaced above and extending over said flat surface, each of said arms having a radially inward facing bearing surface, a jaw, a jaw pivot pin nonrotatably secured to said jaw and having an axially elongated knurled recess near each of its axially opposite ends, and a sleeve of self lubricating high temperature resistant plastic molded into each of said recesses, said sleeves presenting radially outward facing cylindrical bearing surfaces on said jaw pivot pin mating, respectively, with said bearing surfaces on said arms.
 5. The tenter clip of claim 4 wherein the outside diameter of said jaw pivot pin including said sleeves is machined to a uniform diameter.
 6. The tenter clip of claim 4 wherein said high temperature resistant plastic has a melting point greater than 600 degrees F.
 7. The tenter clip of claim 4 wherein said high temperature resistant elastic is polytetraflouraethylene.
 8. A cylindrical pivot pin comprising: a shallow recess formed near each end of said pin, each of said recesses having a radially outward facing cylindrical surface which has been knurled and a high temperature resistant, self lubricating plastic sleeve bonded to each of said roughed up cylindrical surfaces, each of said sleeves presenting a radially outward facing cylindrical bearing surface.
 9. The pivot pin of claim 8 wherein said plastic is polytetraflouraethylene.
 10. The pivot pin of claim 8 wherein the outer diameter of said pin including said sleeves is a uniform diameter. 